Phase-stable receiver



April 5, 1966 Original Filed Nov. 13, 1962 R. E. GRAVES ETAL 3,245,077

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April 5, 1966 R. E. GRAVES ETAL PHASE-STABLE RECEIVER 3 Sheets-Sheet 3Original Filed Nov. l5, 1962 United States Patent O 3,245,077PHASE-STABLE RECEIVER Ross E. Graves, Pacific Palisades, Don M. Jacob,Los Angeles, and Jacob M. Sacks, Palos Verdes Estates, Calif., assignorsto TRW Inc., a corporation of Ohio Original application Nov. 13, 1962,Ser. No. 237,229. Divided and this application Aug. 28, 1964, Ser. No.

6 claims. (c1. 343-7) This invention relates generally to a phase-stablereceiver for use in continuous wave (C.W.) Doppler systems.

This application is a division of copending application Ser. No. 237,229filed November 13, 1962.

One of the major problems in the development of precision C.W. guidanceand tracking systems is the design and implementation of the trackingreceivers. These receivers typically are required to operate over adynamic range of 80 db or more while tracking a vehicle travelling at avelocity up to 40,000 ft./sec. with an acceleration as high as 1,000ft./sec.2. The receivers must generally operate down to signal levels assmall as -140 dbm while tracking a vehicle with the aforementionedcharacteristics. With guidance and tracking systems of the sortcontemplated, position and rate measurements are made by means ofinterferometric techniques, employing system baselines whose lengths mayrange from a thousand feet to a number of miles. Rate measurements aremade by direct use of carrier Doppler data, while position measurementsrequire the use of subcarriers for the resolution of ambiguities in thecarrier phase differences measured by pairs of ground receiving stations(angular data) and in the round-trip phase shift fromground-to-vehicl-toground (range data). One of the preferred methods isto insert the high frequency subcarriers as single sideband subcarrierson the system carrier while the ranging subcarriers and, possibly, thelow frequency angle ambiguity resolving subcarriers may conveniently befrequency (or phase) modulated onto the carrier. In some cases, only onesingle sideband subcarrier may be employed, and this subcarrier may beswept in frequency or stepped discretely in frequency for the purpose ofboth range and angle ambiguity resolution.

Regardless of the particular manner in which the subcarriers aremodulated onto the carrier, the accuracy with which carrier andsubcarrier phase data may be obtained from the tracking receivers is ofparamount importance for the operation of the guidance or trackingsystem. In addition, synchronization of such long baseline systemsrequires the use of radio links along the baseline paths; and thereception of such synchronizing data requires additional phase-stablereceivers. The problem of phasestabilizing an ultra high frequency (UHF)or microwave receiver is rendered extremely difficult by the combinationof requirements, which include large dynamic range, ability to functiondown to a very low signal level, capability of tracking over a widerange of vehicle velocities and for very large vehicle accelerations,high output signal-to-noise ratios, and very high phase stability, allof which must be satisfied simultaneously. It should be observed thatphase accuracy requirements on such receivers may be as stringent as 0.1electrical degree, while accuracies of a few electrical degrees arerequired even in those systems for which the phase data accuracyrequirements are more lenient.

This invention is concerned with techniques for receiver phasestabilization. The receiver design utilizes techniques which areapplicable to any widebase guidance interferometer or to any receivingsystem having comparable requirements. The receiving system iscompatible 3,245,077 Patented Apr'.v 5, 1966 with both UHF and microwavefrequencies; in the microwave case the frequency of the single sidebandsubcarrier might be as high as several hundred megacycles per second.

In this invention the received signals, which may include a carrier, asingle sideband subcarrier, and a synchronizing signal, are each trackedindividually in a separate receiving channel. The signals from the threereceiving channels are combined to yield the output phase data required.The receiver, actually constructed and incorporating the benefits ofthis invention, is capable of tracking the carrier alone down to dbm andboth the subcarrier and the carrier from -40 dbm to -120 dbm, with amaximum source velocity of $40,000 ft./sec. and a maximum sourceacceleration of 30 gs. The required tracking accuracy was 5 degrees forthe carrier and 0.1 degree for the subcarriers. The phase shifts of theoutput signals are kept small by using the principals described in thisinvention.

The receiver does not employ a conventional RF mixer and intermediatefrequency (IF) amplifier with automatic gain control (AGC). Instead,phase shifts in the high gain amplifier stages are minimized byinjecting a voltage controlled oscillator (VCO) signal, or what wouldnormally be the local oscillator signal. This injected signal is fed tothe input of the RF amplifier at a level that is large relative to thereceived signal and with a prescribed audio frequency offset from thereceived signal which is small relative to the bandwidths of theamplifying stages. The term audio frequency as employed here and in thefollowing discussion is used to emphasize that the offset frequency islow relative to the intermediate frequency which would be employed in aconventional receiver for this portion of the frequency spectrum. Invarious embodiments this offset frequency could range from as low asseveral kilocycles to as high as several megacycles. The sum of thesetwo signals, after amplification in the RF amplifier, is detected toobtain anl audio frequency beat note (30 kc. in the preferredembodiment). The audio beat note (30 kc.) contains the desired phaseinformation originally on the incoming signal, which is furtheramplified prior to phase detection with respect to the output of areference audio oscillator (30 kc.) to obtain an error signal to controlthe VCO. For certain applications it will prove desirable to heterodynethe output of the RF amplifier to an intermediate frequency for furtheramplification prior to detection, or even to delete the RF amplifierentirely and perform injection of the VCO signal immediately ahead ofthe mixer employed to heterodyne the composite signal to theaforementioned intermediate frequency. Use of an IF amplifier in themanner described retains the basic advantages of the RF referenceinjection technique. By this technique, any phase shifts which areintroduced in the received signal by the RF amplifier are compensated byvirtually identical phase shifts on the injected RF reference signal,which is offset by the prescribed audio frequency (30 kc.). The resultis that any phase error introduced on the received signal in the RF andin the IF arnplifier, if employed, is substantially removed by beatingthe signal against the injected reference in the detector. In thepreferred embodiment it was possible to increase the signal to a level 6db above the injected reference before significant phase shift occurred,which permitted attainment of additional dynamic range in the receiverfor strong signals.

In addition, the injected reference signal power is large relative tothe total noise power in the bandwidth presented to the detector, withthe result that the signal-tonoise ratio is not degraded bynoise-cross-noise products generated in the detector.

Further objects and advantages of this invention will be made moreapparent by referring now to the accompanying drawings wherein:

FIG. 1 is a block diagram of a long base line C.W. Doppler system forlocating and tracking objects in space;

FIG. 2 is a block diagram of a receiver illustrated in the system ofFIG. 1;

FIG. 3 illustrates the frequency spectrum of the received signals andthe injected signals used in the receiver of FIG. 4; and

FIG. 4 is a more complete block diagram of the system illustrated inFIG. 2.

The invention is more properly concerned with a phase stable receiver;however, in order to more fully appreciate the application and use ofthe receiver the invention is also described in connection with a C.W.system requiring a phase stable receiver.

Referring now to FIG. 1, there is shown a continuous wave Doppler systemcomprising a transmitted and a plurality of remote receivers 11, 12, and13. The continuous wave Doppler transmitter 10 is arranged to transmit aC.W. signal to a moving object 14. The moving object may be a satelliteor missile and may include a coherent transponder, which is one thatreceives the transmitted signal from transmitter 10 and rebroadcaststhis signal with a frequency offset coherent with the received signal.Alternatively, the moving object may be passive in nature so that itwill simply reflect any transmitted signal that impinges upon itssurface. The reected or coherent transmitted signal from the movingobject 14 is adapted to be received by all of the receivers 11, 12, and13. The transmitter 10 is also adapted to transmit a synchronizing(SYNC) signal to all the receivers 11, 12, and 13, which SYNC signal isphase coherent with the C.W. signal transmitted to the moving object 14.The output signals from each of the receivers 11, 12, and 13, togetherwith frequency and timing (phase) information from the transmitter 10,are fed to a data processor 15, consisting of analog and digital dataextraction equipment and of computers and associated equipment, which isresponsive to the output of said receivers for determining range andbearing information of the moving object 14. Each of the receivers 11,12, and 13 is identical and is arranged to compare the phase of thecarrier signal received from the moving object 14 with the SYNC signalreceived directly from the transmitter 10. The phase change in each ofthe receivers is an indication of the relative movement of the movingobject 14. The three receivers in this configuration are known as aninterferometer and by themselves will produce sutiicient information todetermine pairs of range differences. In the case where the distance tothe moving object 14 is large relative to the separations (baselines)between the receivers 11, 12, and 13, these range differences aresubstantially equivalent to angular position (bearing) information. Thecombination of the transmitter 10 with any of the receivers will produceranging information which, together with the range differenceinformation from the three receivers connected as an interferometer,will produce range and bearing information sufficient to track an objectin space.

In one embodiment the transmitter 10 transmits a CNV. carrier signal of400 me. with a subcarrier of 404 mc. Each of the receivers 11, 12, and13 receives reections of the 400 mc. carrier and 404 mc. subcarrier fromthe moving object 14. In this case, the phase data of interest are thephase of the 400 mc. carrier and the phase of the 4 mc. subcarrier whichis obtained by demodulating the 404 mc. sideband with respect to the 400mc. carrier. In the following discussion, it will be convenient to referto both the 4 mc. and the 404 mc. signals as subcarriers, with theunderstanding that the 404 mc. signal is to be thought of as obtained bysingle-sideband modulation of the 4 mc. signal onto the 400 mc. carrier.

The phases of the 400 mc. received carrier signals and the phases of thereceived 4 mc. subcarrier signals are compared in pairs in the dataprocessor 15 to determine the range differences previously discussed.Similarly, comparison of the phase of the 4 mc. subcarrier received fromthe moving object 14 by receiver 12 with the phase of the 4 mc.subcarrier transmitted from transmitter 10, effected in the dataprocessor 15, permits the measurement of (ambiguous) ne range to themoving object 14. The SYNC signal generated by transmitter 10 was xed at2.31 mc. below the carrier signal of 400 rnc. Each of the receivers isarranged to track the 400` mc. carrier signal, the 4 mc. modulating(subcarrier) signal, and the 2.31 mc. SYNC signal. In a secondembodiment of the system described in FIG. l, the moving object 14contained a 400 mc. transmitter and 4 mc. modulating source. Thetransmitter 10 was arranged to transmit a 17/16 400 mc. CW. signal tothe moving object 14, which then received the 17/16 400 mc. carriersignal and phase coherently retransmitted the 400 mc. carrier that wasmodulated by the 4 mc. source located in the moving object. In thisembodiment the ground based receivers 11, 12, and 13 performed the sameoperations as described for the rst embodiment. In this case, thereceived signals from the moving object 14- were at a substantiallyhigher signal level, thereby materially irnproving the signal-to-noiseratio at the receiver. This system is considered more desirable for theguidance and tracking of friendly objects where suitable transpondingequipment may be installed. The principals of the phase stable receiverto be described are the same in either case. It should be observed,however, that when the 4 mc. signal is obtained from a source in themoving object 14, the resultant received 4 mc. phase data cannot beemployed for range measurement, as a consequence of the lack of a 4 mc.phase reference in the ground system, even though they are completelysatisfactory for measurement of range differences.

The phase-stable receiver receives a carrier signal and modulatingsignal from the moving object and measures the phase change in themodulating signal by using the carrier signal as a reference. This phaseinformation when received from at least three different ground stationsis suicient to give range difference information. In the preferredembodiment a SYNC signal containing the phase information of thetransmitted carrier is also sent to all ground stations in order toobtain range and angle information. By using a carrier frequency of 400mc. and a modulating frequency of 4 mc., it can be shown that every 360degree phase change of the 400 mc. carrier is equivalent to 1.23 feetand that every 36() degree phase change of the 4 mc. modulating signalis equal to 123 feet. It is possible by conventional techniques tomeasure the phase of the 400 mc. carrier signal to within 5 degrees andthe phase of the 4 mc. subcarrier to approximately 1/2 degree. Theaccuracies claimed for the system will therefore approximate 0.018 feetfor the carrier signal and 0.17 feet for the subcarrier signal. In otherwords, the 4 mc. subcarrier signal provides unambiguous rangeinformation for every 123 feet measured by the 400 rnc. carrier signal.Additional range ambiguity can be resolved by amplitude modulating thecarrier with a suiciently low frequency depending upon the maximum rangeexpected. For example, using a 137 cycle signal will provide anunambiguous range of 40,000 feet,

Referring now to FIG. 2, there is shown a phase-stable receiver adaptedto receive a 400 mc. carrier and a 404 mc. subcarrier signal. Thereceiver comprises a carrier loop 20, a subcarrier loop 21, and asynchronizing loop 22. The improved phase characteristics claimed forthe phase-stable receiver are believed due primarily to the use of an RFinjected reference signal whose frequency is offset by an audiofrequency from the frequency of the received signal. The offset audiofrequency was selected in the preferred embodiment to be 30 kc. Theinjected reference signal is amplified together with the carrier signaland detected to obtain the resultant audio frequency between the carriersignal and the injected reference signal. This technique reduces theeffect of phase shifts within the receiver to a very small value sincethe phase information is contained in the 30 kc. audio offset signal.IIn the carrier loop 20, the received 400 mc. carrier signal fromantenna 23 is fed to an adder 24 where the carrier signal and theinjected reference signal from VCO 25 are combined. Both the 400 rnc.carrier and the injected reference signal at a frequency of 400 mc.-l-30kc. are fed to and amplified in an RF amplifier 26. The output of RFamplifier 26 is fed to an envelope detector S0. The detected 30 kc. beatnote is feci to a phase detector 27 where the phase of the 30 kc. audiooffset is compared with a reference signal generated by a free running30 kc. oscillator 28. The output of the phase detector 27 is a D.C.signal which is fed to a filter and amplifier 29 and, then used tocontrol the frequency of the 400 mc. VCO 25. The loop circuit ljustdescribed generates an output signal that is offset from the incomingcarrier signal by 30 kc. In other words, the carrier loop circuit 20continuously tracks the difference or 30 kc. audio offset signal,thereby insuring an injected signal that has a fixed frequency offsetfrom the incoming carrier signal.

The subcarrier loop 21 comprises a mixer stage 29, which is arranged toreceive the output signal from the VCO 25 and an output signal from a 4mc. VCO 30. The output from mixer 29 represents the injected referencesignal for the subcarrier loop 21 and is fed to an adder 31, where theinjected reference is combined with the received 404 rnc. subcarrierfrom antenna 23. The output from adder 31 is amplified by an RFamplifier 32 and consists at least of the incoming subcarrier at 404 mc.and the injected reference signal at 400.0304-4 rnc. or 404.30 mc. Itcan be seen therefore that in the subcarrier loop 21 RF amplifier 32 isactually amplifying two signals that are only 30 kc. apart, therebyminimizing the phase shift in the loop to a very small value. The outputof the RF amplifier 32 is fed to an envelope detector 53. The detected30 kc. beat note is fed to a phase detector 33, which compares the phase-of the detected 30 kc. beat note withthe same reference signalgenerated by the 30 kc. oscillator 28. The output of phase detector 33is fed to a filter and amplifier 34, the output of which is used tocontrol the frequency of the 4 mc. VCO 30. The output 'of the subcarrierloop 21 is taken from the output of the 4 rnc. VCO 30, which containsthe necessary phase information.

For those systems requiring only relative angle infor-mation, it is onlynecessary to compare the phase of this 4 mc. signal against thatreceived by other receiving stations to thereby obtain range differencedata for the moving object. For those systems requiring specificlocation of the moving object, it is necessary to compare the phase ofthe 4 mc. output signal with the phase of the transmitted subcarriersignal to thereby obtain range information. The combining of the rangeinformation together with range difference data will supply thenecessary information to locate the moving object in space.

The SYNC carrier loop 22 supplies the phase information which is relatedto the transmitted 400 mc. carrier signal for providing the system withthe means for obtaining range and angle information. The 400-2.31 mc.SYNC signal is received from the transmitter by -rneans of antenna 35and fed to adder 36. The injected reference signal is composed of afirst signal from the output of the 400 rnc. VCO 25, which is mixedwith'the output of a 2.31 mc. VCO 37 in mixer 38. The signal from theVCO 25 is actually 400.030 rnc., which is mixed with the 2.31 mc. outputfrom VCO 37 in mixer 38. The injected frequency will therefore be 397.72mc., which will be added to the received 397.69 rnc. SYNC signal, whichsignals are amplified by RF amplifier 39. The output of the RF amplifier39 is fed to an envelope detector 56. The

detected 30 kc. beat note is fed to a phase detector 40 where the 30 kc.beat note frequency is phase compared with a reference signal receivedfrom the same 30 kc. oscillator 28. The output of the phase detector 40is fed to a filter and amplifier 41, the output of which controls thefrequency of the 2.31 mc. VCO 37. The 2.31 mc. output frequency containsphase information of the 400 me. carrier and when used in conjunctionwith the 4 mc. VCO 30 will produce range and bearing information. Areview of the carrier loop 20, the subcarrier loop 21, and the SYNC loop22 will show that all three loop circuits are locked together andactually lock on the 30 kc. audio offset frequency.

Since each of the defined loop circuits actually tracks a 30 kc. signal,it is important that the phase shift of each loop circuit be the same orvery close to being the same as dictated by the ultimate requirements ofthe overall system. By using appropriate loop gains in the carrier loop20, subcarrier loop 21, and the SYNC loop 22, the dynamic phase errorresulting from acceleration of the moving object may be made nearlyidentical in all of the defined loop circuits. By keeping the phaseerror the same for all loops, no resultant error will appear in theoutput signal. Phase differences between the loops, caused bydifferential phase shifts in the different tracking loop lter circuits,phase detectors, and filter amplifiers, can be reduced by phasedetecting circuits 42 and 43. Phase detecting circuit 42 compares thephase of the 30 kc. beat note signal generated in the subcarrier loop 21with the phase of the 30 kc. beat note signal generated in the carrierloop 20 by feeding the signals directly to a differencer 44. The outputof the differencer 44 is fed to a phase comparator and filter 45, wherethe signal is compared with a reference 30 kc. signal generated by the30 kc. oscillator 28. The output of the phase comparator 45 is fed tothe filter and amplifier 34 in the subcarrier loop 21, thereby insuringthat any phase difference of the 30 kc. beat note signal generated inthe carrier loop 20 will be the same as that of the 30 kc. beat notesignal generated in subcarrier loop 21. In a similar fashion, the phaseof the 30 kc. beat note signal generated in the SYNC loop 22 is comparedwith the phase of the 30 kc. beat note signal generated inthe carrierloop 20 by means of phase detecting circuits 43. The 30 kc. beat notesignal from carrier loop 20 and the 30 kc. beat note signal generated inthe SYNC loop 22 are fed to a differencer 46. The output of thedifferencer 46 is fed to a phase comparator and filter 47, where thesignal is compared against a reference signal generated by the 30 kc.oscillator 28. The output of the phase comparator 47 is fed to thefilter and amplifier 41 in the SYNC loop 22, thereby insuring that thephase of the SYNC loop and the carrier loop will be the same.

The phase detecting circuits 42 and 43 are preferably commutated phasedetectors having high gain, low error and low drift. Circuits of thiscaliber have been dis-l closed and claimed in copending application,entitled Phase Stable Limiter Amplifier, Serial No. 237,267, Jacob M.Sacks, applicant, now abandoned. The feed, back signal from phasedetecting circuit 42 could have been alternatively directed to thefilter and amplifier y29 in the carrier loop 20. This feedback resultsin a reduction of the initial phase error obtained without the phasedetecting circuits by a factor which depends on the gain of the phasedetecting circuit.

Referring now to FIG. 3, there is shown a frequency spectrum unaffectedby Doppler shift, illustrating the rereived carrier signal at 400 mc.and the subcarrier displaced from the carrier by 4 rnc. Also shown isthe received SYNC signal which is displaced from the carrier on the lowside by 2.31 mc. As mentioned previously, the injected reference signalsfor the carrier signal, the subcarrier signal and the SYNC signal areeach shown displaced from their respective received signal by the audiooffset frequency of 30 kc. The relative amplitudes un t of the injectedreference signals are shown in an exaggerated condition to more fullyillustrate the increased amplitude of the injected reference signalsover the corresponding received signals.

Referring now to FIG. 4, there is shown a block diagram illustrating inmore detail the system described in FIG. 2. In describing the moredetailed block diagram of FIG. 4, similar numbers used in connectionwith FIG. 2 will be used whenever the complete block performs the samefunction as described in FIG. 2. However, wherever additional blockshave been added to more fully describe the function of the system, newnumbers will be assigned and used.

In both FIGS. 2 and 4, the 400 rnc. and 404 mc. signals are illustratedas being received and separated prior to addition of the injectedreference signals for purposes of illustration only and to helpunderstand the invention. It should be noted that whenever two receivedsignals are so close together that the indicated power splitter willproduce a 3 db loss in the desired signal component, thereby degradingthe noise performance of the receiving system, this loss may beeliminated by inserting the injected reference signals before any powersplitting and then amplifying as shown before envelope detecting thesignals.

The incoming carrier signal fed to adder 24 is actually a frequencyvarying signal having phase information which may be represented aswot"i*lo The injected reference signal generated by the VCO 25 is lockedto the incoming carrier signal and offset therefrom by 30 kc. aspreviously described and may be mathematically represented as:

Both the incoming carrier signal and the injected reference signal areamplified by the RF amplifier 26. The difference signal containing thephase information is detected by an envelope detector 50. The output ofthe envelope detector 50 is the 30 kc. offset signal which is fed to afilter 51 to remove the high frequency carrier components. The filtered30 kc. signal from filter S1 is fed to a limiter 52 for generating a 30kc. signal at a given amplitude. In the embodiment described the definedlimiters 52, 55, and 58 must be extremely phase stable with respect toamplitude. Details of a limiter circuit having the necessary phasestability are described and claimed in a copending application, SerialNo. 237,267, filed November 13, 1962, and assigned to the same commonassignee. It should be understood, however, that while the limitertechnique for performing automatic gain control on the 30 kc. signalsrepresents the preferred embodiment, there are many other techniquesthat are also suitable. For example, the simple expedient of using anAGC audio amplifier having a phase shift that varies a small amount withchanges in AGC bias. Phase detector 27 compares the phase of thereference 30 kc. oscillator 28 with the limited 30 kc. signal fromlimiter 52 and generates a pulsating D.C. signal of the proper amplitudeand sense as a function of the phase difference between the input 30 kc.signals. The pulsating D.C. output from the phase detector 27 is fed tothe filter and amplifier 29 which generates a D.C. signal which is usedto control the output of the 400 mc. VCO 25. The output of the VCO 25will be the injected reference signal having a frequency of 400.030 mc.and a phase given by:

(wofhwsof) l- (41o-F9530) A review of the subcarrier signal trackingloop 21 will show that the 404 rnc. subcarrier identified as wst-I-s isfed to the adder 31 together with the injected reference signal which is30 kc. offset from the subcarrier and identified asfwst-l-wm-j-(qs-j-pso). In a similar fashion, as described for thecarrier loop 20, both signals are amplified by an RF amplifier 32 andfed to an envelope detector 53. The detected 30 kc. signal from theenvelope detector S3 is fed to a filter 54, which removes thc highfrequency carrier components and then is fed to a limiter 55. The outputof the limiter 5S is compared with the output of the 30 kc. oscillator28 in the phase detector 33. The pulsating output from the phasedetector 33 is fed to a filter and amplifier 34, which generates a D.C.signal varying in amplitude and sense as a function of the difference inphase between the reference signal and the output of limiter 55. ThisD.C. signal controls the 4 mc. VCO 30. The 4 mc. output from VCO 30 isfed to the mixer 29, which combines the 4 mc. signal with the 400.030mc. output from the VCO 25, located in the carrier loop 20. The injectedreference signal fed to the adder 31 is therefore a 404.030 rnc. signal,which is 30 kc. offset from the received 404 mc. subcarrier signal,which is also fed to the adder 31.

The SYNC loop 22 is very similar to the subcarrier loop 21 in that theSYNC signal is received as a subcarrier that is 2.31 mc. removed on thelow side of the 400 mc. carrier. This received signal is fed to theadder 36, which also receives the injected reference signal from themixer 38. Both signals are amplified .by the RF amplifier 39 and fed toan envelope detector 56 where the difference signal of 30 kc. isextracted. This signal is fed to a filter 57 and a limiter 58 in thesame fashion as previously described. The phase detector 40 generates apulsating D.C. signal in response to the phase difference between the 30kc. signal received from the limiter 58 and the 30 kc. referenceoscillator 28. The pulsating D.C. signal is fed to the filter andamplifier 41, the output of Iwhich controls the 2.31 mc. VCO 37. This2.31 mc. signal is combined with the 400.030 mc. signal from the VCO 25and the carrier tracking loop 20 in the mixer 38. The resulting injectedreference signal of 397.72 mc. is combined in the adder 36 with thereceived SYNC signal of 397.69 mc.

The advantage of the defined receiver is that any phase shiftsintroduced into the carrier, subcarrier, and SYNC signal receivingchannels due to Doppler shifts or signal level variations in the RFamplifier and limiter amplifiers, which are of the same magnitude forboth channels, will not appear as phase errors in either the 4 mc.subcarrier output signal or in the 2.31 mc. carrier output signal. Afurther advantage is that the dynamic phase error due to acceleration ofthe moving object will tend to cancel between the carrier and subcarrierchannels (or carrier and synchronizing channels). The dynamic phaseerror which can result from acceleration of the moving object can beminimized by the proper selection of reference gains in the carrier loopcircuit and the subcarrier loop circuit. By making the dynamic phaseerror exactly the same for each loop, no resultant error will appear inthe output signal. In an idealized situation the phase error of eachloop can be made identical; however, it is known that active elements inthe loop filters, phase detectors and loop filter amplifiers will causedifferential phase shifts. It is the purpose of phase detecting circuit42 to compare the phase shift in the 30 kc. signal in the carrier loop20 with the phase shift of the 30 kc. signal in the subcarrier loop 21and in response thereto generate an additional D.C. signal of properamplitude and sense to thereby make the phase-y shift of the subcarrierloop 21 the same as the phase shift. in the carrier loop 20. The phasedetecting circuit 43 per-` forms the same function with respect to thecarrier loopl 20 and the SYNC loop 22, thereby insuring that all threeloop circuits will have substantially the same dynamic lag phase error.in this manner dynamic lag errors in the outputs on the 4 mc. VCO 30 andthe 2.31 mc. VCO 37 `are* substantially eliminated.

The phase comparison circuit 42 comprises a differencer 44 whichreceives a first input consisting of the limited 30 kc. signal from theoutput of limiter 52, located in the carrier loep 20, and the limited 30kc. signal from the limiter 55, located in the subcarrier loop Z1. Thetwo 30 kc. signals which are of approximately equal magnitude areactually added in antiphase by the differencer 44 prior to beingfiltered in a bandpass filter 59. The output of the filter 59 is thenamplied by an amplifier 60 and phase detected with respect to the outputof the 30 kc. oscillator 28 in a phase detector 61. Since the phasedetector is only sensitive to a voltage that is in phase quadrature withrespect to the initial input signals, it can be shown that thequadrature voltage will arise only when a phase difference existsbetween the two input signals and will not exist when there is only anamplitude difference. In other words, should the angle between theoriginal input voltage and the phase detector reference voltage not be9() degrees, then any difference in the amplitudes of the signalcomponent from limiter 55 in the subcarrier loop 21 and that fromlimiter 52 in the carrier loop will produce only a second order effecton the output error signal from phase detector 61. In the preferredembodiment, the defined second order effect is reduced or eliminated bycontrolling the output amplitude of the last stage of the limiter 55with an AGC circuit which operates to maintain the amplitude of thesignal component of the output of this limiter substantially equal tothe amplitude of the signal component of the output of limiter 52.Alternately, this defined second order effect can be reduced oreliminated by means of external circuitry for servoing the phase errorso that it is always 90 degrees with respect to the phase detectorreference voltage. The pulsating D.C. signal from the phase detector i61is fed to a filter 62 which generates a D.C. signal Varying in amplitudeand sense as a function of the phase difference between the carrier loop20 and the subcarrier loop 21. This D.C. signal is fed to the filter andamplifier 34 where it is combined with the D.C. control signal generatedby the subcarrier loop 21, which is ultimately used to control the 4 mc.VCO 30. In this manner the dynamic phase error of the subcarrier loopwill be the same as the phase error of the carrier loop. The phasecomparing circuit 43 is similar in function to the phase detectingcircuit 42. The phase of the carrier loop 29 is compared with the phaseof the SYNC loop 22 in order to generate an error signal to therebycause the SYNC loop to have the same phase lag as the carrier loop 20.The 30 kc. signal from the output of limiter 52 in the carrier loop 20and the 30 kc. signal from the output of limiter 58 in the SYNC loop 22are both fed to the differencer 46 located in the phase comparingcircuit 43. The difference between these two signals represents an errorsignal indictating a phase difference between the output of the carrierloop and that of the SYNC loop. This error signal is fed to a filter 63,which in turn feeds an amplifier 64. The output of amplifier 64 iscompared with the output of the 30 kc. reference oscillator 28 by meansof phase detector 65. The pulsating D.C. output from the phase detector65 is fed to a filter 66 which generates a D.C. signal varying inamplitude and sense according to the phase difference between the outputof the carrier loop and that of the SYNC loop. The output of the filter66 is fed back into the filter and amplifier 41, located in the SYNCloop 22. This feedback D.C. signal is combined with the D.C. signalcontrolling the 2.31 mc. VCO 37 to thereby correct the phase of the SYNCloop output from the 2.31 mc. VCO 37 for dynamic lag phase errors in thecarrier loop.

Considerable reduction in the effect of drift in the output of the phasecomparing circuits 42 and 43 is obtained as a result of the additionalgain from amplifiers 60 and 64, which can be inserted in the signal pathprior to the phase detectors without causing saturation. Insertion ofthis .additional gain is possible because of the reduction in inputamplitude which is obtained by adding the signals from the limiters S2and 55 in antiphase and by adding the signals from the limiters 52 and58 in antiphase. In addition, low drift and very small noise unbalanceof the phase detectors can be obtained by means of the commutationtechn-ique described in said copending applica tion. The 30 kc.reference of the commutated phase detector may be inverted in phaseevery 16 cycles, thereby producing a 940 c.p.s. error signal at theoutput of the phase detector. The peak-to-peak amplitude of this errorsignal is proportional to the phase error between the two initial inputsignals from the limiters, and the A.C. compone-nt of the phase detectoroutput has one of two phases, depending on the sign of the phase errorbetween the signals from the two limiters. The A.C. component of thephase detector output is amplified and then decommutated in asynchronous clamping circuit. The complete circuit, including the 30 kc.reference commutator, the simple phase detector, and the decommutator isreferred to as the commutated phase detector. This commutated phasedetector is a very sensit-ive device having extremly low drift. Becauseof the high gain employed, the output signal will saturate for only afew degrees of phase difference in the input signals. However, only afew degrees phase error will ever occur at the input to the commutatedphase detectors during normal operation of the signal combiner loops, asembodied in the phase comparing circuits 42 and 43.

The practice of adding the two signals at the outputs of the limiters inantiphase and phase detecting the resultant sum with respect to theoutput o-f the audio reference oscillator, rather than merelyphase-detecting the output of one of the limiters with respect to theoutput of the other limiter, is employed in order to reduce the level atwhich thresholding occurs in the phase detector 61 or 65. If the outputof one limiter 52 were merely to be phase detected with respect to theoutput of the other limiter 55, the action of the phase detector wouldgenerate noise-cross-noise products, caused by the noise in one channelbeating with that in the other channel, which would cause the phasedetector to threshold for input signal-to-noise ratios of the order ofunity or somewhat below. Moreover, in this case it would not be feasibleto perform substantial additional filtering in either or both of the twochannels prior to phase detection because of the phase shifts and driftswhich would be caused by the required audio filters.

In the present invention the performance of the phase detecto-r isimproved since the noisy signal obtained by adding the outputs of thepair of limiters in antiphase is phase detected with respect to anoise-free signal from the reference audio oscillator. This fact impliesthat, as long as the level of the reference signal is large relative tothe total level of the signal-plus-noise in the other phase detectorinput channel, thresholding caused by noisecross-noise products will notoccur. In addition, after the two noisy signals have been added inantiphase, it is feasible to employ additional gain before phasedetection and also to use a narrowband filter to improve thesignalto-noise ratio presented to the phase detector in the signalchannel. Any phase shift introduced by the narrowband filter on thesignals from the two limiter-amplifiers will be identical and, hence,will produce negligible resultant offset in the phase detector as longas the amplitudes of the signal components in the limiter outputs arewell balanced.

This completes the description 0f the embodiment of the inventionillustrated herein. However, many modifications and advantages thereofwill be apparent to persons skilled in the art without departing fromthe spirit and scope of this invention. Accordingly, it is desired thatthis invention not be limited to the particular details of theembodiment disclosed herein, except as defined by the appended claims.

The embodiments of the invention in which an eX- clusive property orprivilege is claimed are defined as follows:

1. In a system having at least two phase-locked loop circuits in whicheach loop circuit is adapted to phase lock on the difference between areceived signal and an E i i injected signal differing in frequency byan arbitrarily selected offset frequency comprising:

means in each of said loops for detecting said offset frequency, meansfor adding said detected signals in phase opposition to produce an errorsignal, and means for controlling the phase of one of said signals withsaid error signal to minimize said error signal. 2. In a system havingat least two phase-locked loop circuits in which each loop circuit isadapted to phase lock on the difference between a received signal and aninjected signal differing in frequency by an arbitrarily selected offsetfrequency comprising:

means in each of said loops for detecting said reference signal, meansfor adding in phase opposition said detected signals to produce an errorsignal, means for amplifying said error signal, and means for changingthe phase of said detected signal in one of said loops with a componentof said amplified error signal to minimize said error signal. 3. A phasecomparing system comprising: oscillator means for generating asubstantially constant frequency reference signal, at least two loopcircuits each comprising means for receiving a frequency varying signal,means responsive to said reference signal for generating an injectionsignal at a frequency differing from said frequency varying signal bysaid reference signal frequency, means for adding said carrier signaland said injection signal, means for detecting said reference signalfrom said added signals, `means responsive to the phase differencebetween said `detected reference signal and said reference signalreceived directly from oscillator means for controlling said injectionsignal frequency, means for `adding said detected reference signals fromsaid loop circuits in phase opposition with each other to produce anerror signal, and means for controlling one of said loop circuits withsaid error signal to minimize said error signal. 4. A phase -comparingsystem comprising: an -oscillator for generating a substantiallyconstant frequency reference signal, at least two loop circuits eachcomprising Ameans for receiving a frequency varying carrier signal,mean-s responsive to said reference signal for generating an Iinjectionsignal at a frequency differing from said carrier signal by saidreference signal frequency, means for combining said carrier signal andsaid injection signal, means for detecting said reference signal fromsaid combined signal,

means responsive to the phase difference between said detected referencesignal and said oscillator generated reference signal for controllingthe frequency of said injection signal,

means for adding said detected reference signals from said loop circuitsin phase `opposition to produce a first error signal,

`means for amplifying said first error signal,

means for comparing the phase of said amplified first error signal withthe phase of said oscillator generated reference signal to produce asecond error signal, and

means for controlling one of said loops with said second error signal tominimize said first error signal.

5. A phase comparing system comprising:

an oscillator for generating a substantially constant frequencyreference signal,

at least a first, a second, and .a third loop circuit, each comprisingmeans for receiving a frequency varying signal, means responsive to saidreference signal for generating an injection signal at a frequencydiffering from said signal by said reference signal frequency,

means for adding said carrier signal and said injection signal,

means for detecting said reference signal from said combined signal,means responsive to the phase difference between said detected referencesignal and said oscillator generated reference signal for controllingsaid injection signal,

means for adding said detected reference signals from said first andsecond loop circuits in phase opposition with each other to produce afirst error signal,

means for controlling said second loop circuit with said first errorsignal to minimize said first error signal,

`means for adding said detected reference signals from said first andlsaid third loop circuits in phase opposition With each other to producea second error signal, and

means for controlling said third loop circuit wi-th said second errorsignal to minimize said second error signal.

6. A system according to claim 5 in which said first and second phaseloops are adapted to receive a carrier and subcat'rier signal from amoving object, and said third loop is adapted to .receive asynchronizing signal from a ground source containing phase informationabout said carrier signal.

No references cited.

LEWIS H. MYERS, Primary Examinez'.

CHESTER L. JUSTUS, Examiner.

R. D. BENNETT, Assistant Examiner.

1. IN A SYSTEM HAVING AT LEAST TWO PHASE-LOCKED LOOP CIRCUITS IN WHICHEACH LOOP CIRCUIT IS ADAPTED TO PHASE LOCK ON THE DIFFERENCE BETWEEN ARECEIVED SIGNAL AND AN INJECTED SIGNAL DIFFERING IN FREQUENCY BY ANARBITRARILY SELECTED OFFSET FREQUENCY COMPRISING: MEANS IN EACH OF SAIDLOOPS FOR DETECTING SAID OFFSET FREQUENCY, MEANS FOR ADDING SAIDDETECTED SIGNALS IN PHASE OPPOSITION TO PRODUCE AN ERROR SIGNAL, ANDMEANS FOR CONTROLLING THE PHASE OF ONE OF SAID SIGNALS WITH SAID ERRORSIGNAL TO MINIMIZE SAID ERROR SIGNAL.